In a power converter such as an inverter, a gate circuit of a switching element for driving a load is connected in series with a main electric power source, and a current direction of the load is switched by a bridge circuit, for example.
FIG. 9A shows a configuration example of the power converter. This figure illustrates an example that a full bridge of switching elements 101A to 101D for switching connection between a driving source 103 and a load 104.
The switching elements 101A to 101D are controlled on and off, respectively by the gate driver circuits 102A to 102D.
The gate driver circuit 102A and the gate driver circuit 102C control on and off of the switching element 101A and the switching element 101C, according to high-side input signals S-A and S-C, and the gate driver circuit 102B and the gate driver circuit 102D control on and off of the switching element 101B and the switching element 101D, according to low-side input signals S-B and S-D, whereby a current direction toward the load 104 is switched.
In this gate driver circuit, according to the on/off state on both the high side and the low side, the potential of the high-side switching elements 101A and 101C is liable to floating, causing fluctuations of reference potential, though there is no fluctuation of the reference potential in the low-side switching elements 101B and 101D. Therefore, there is a difference between the high-side reference potential and the low-side reference potential, and this may cause a short circuit.
In order to avoid this short circuit, it is necessary to render the reference potential of the high-side and low-side switching elements as floating potential. The input signals S-A to S-D to the gate driver circuits 102A to 102D for controlling on/off of the switching elements 101A to 101D, respectively, are required to be isolated electrically, so as to render the reference potential of the switching elements as floating.
FIG. 9B shows a configuration example of a power converter that employs isolation devices. In this example, a half bridge of the switching elements 101A and 101B switches the connection between the driving source 103 and the load 104.
On the input side of the gate driver circuits 102A and 102B, the isolation devices 105A and 105B are connected respectively, and the isolation devices 105A and 105B provide electrical isolation between the input signals S-A and S-B and isolated drive signals Siso. There are known a photo coupler or a pulse transformer, as the isolation device for electrically isolating input signals in the gate driver circuit (Patent documents 1 and 2).
The pulse transformer is used as a transformer for driving the gate circuit, and it is also referred to as a gate-driver transformer.
FIGS. 10A and 10B show an example of a circuit that employs the photo coupler. In the drive circuit as shown in FIG. 10A, gate driver circuits 102A and 102B for driving the switching elements 101A and 101B, respectively, are connected to the photo couplers 105A and 105B, whereby isolation is provided between the input signal S-A and the isolated drive signal Siso-A, and between the input signal S-B and the isolated drive signal Siso-B.
It is to be noted that the switching element 101 such as an RF MOS-FET may be combined with the gate driver circuit 102 such as an FET driver, so as to constitute an RF-MOS module which is built in the driver.
FIG. 10B illustrates drive signal waveforms. The reference numeral (b-1) indicates the high-side input signal S-A, the reference numeral (b-3) indicates the high-side isolated drive signal Siso-A, which is obtained by the isolation according to the photo coupler. The reference numeral (b-2) indicates the low-side input signal S-B, and the reference numeral (b-4) indicates the low-side isolated drive signal Siso-B, which is obtained by the isolation according to the photo coupler.
FIGS. 11A through 11C show a circuit example that provides isolation by using a gate-driver transformer (pulse transformer). In the drive circuit as shown in FIG. 11A, a free-magnetization transformer circuits 106A and 106B are connected respectively to the gate driver circuits 102A and 102B for driving the switching elements 101A and 101B, so as to provide isolation between the input signal S-A and the isolated drive signal Siso-A, and between the input signal S-B and the isolated drive signal Siso-B. Also in the circuit example as shown in FIG. 11A, similar to the circuit example as shown in FIG. 10A, the switching elements 101A and 101B such as RF MOS-FETs may be combined, respectively with the gate driver circuits 102A and 102B such as FET drivers, so as to constitute RF-MOS modules built in the driver.
The free-magnetization transformer circuits 106A and 106B as shown in FIGS. 11A through 11C are isolation circuits, each made up of a forward-type gate-driver transformer (pulse transformer), and an FET 106b is connected in series with the primary coil of the gate-driver transformer (pulse transformer) 106a. Furthermore, a series circuit of a rectifier diode and resistance is connected to the primary coil in parallel, and a rectifier circuit 106d is connected to the output end of the secondary coil side.
In the free-magnetization transformer circuit 106, the transformer is excited only in one way, and energy accumulated in the coils is released during a period when the switching element is OFF, thereby resetting the magnetized state. The series circuit of the rectifier diode and the resistance, being connected in parallel with the primary coil, constitutes a reset circuit (snubber circuit) 106c. 
In the example here, resetting of the magnetized state of the coils that have been magnetized is performed, by spontaneously releasing the energy accumulated in the coils. Resetting of the magnetization of the coils is performed spontaneously without external actions, and therefore it is referred to as “free magnetization”.
FIG. 11B shows drive signal waveforms. The reference numeral (b-1) indicates the high-side input signal S-A, and the reference numeral (b-2) indicates the low-side input signal S-B. The reference numeral (b-3) indicates the high-side isolated drive signal Siso-A, which is obtained by isolation by the gate-driver transformer, and the reference numeral (b-4) indicates the low-side isolated drive signal Siso-B, which is obtained by isolation by the gate-driver transformer.